1. Field of the Invention
The present invention relates to an abnormality diagnosis device for a wide range air-fuel ratio sensor for detecting a air-fuel ratio in a linear manner ranging from rich mixture to lean mixture, including a stoichiometric air-fuel ratio from oxygen concentration in exhaust gas from an engine.
2. Description of Related Art
Conventionally, as technology for carrying out abnormality diagnosis for a wide range air-fuel ratio sensor, technology is known for, when the engine is in a predetermined operating state (steady state), carrying out prescribed air-fuel ratio control to measure a diagnostic value relating to response of the air-fuel ratio sensor, and determining an air-fuel ratio abnormality when this diagnostic value exceeds an abnormality determination value.
However, in order to obtain a highly precise diagnostic result using this type of abnormality diagnosis it is preferable to carry out the final decision as to whether or not the air-fuel ratio sensor has an abnormality based on diagnostic values that have been continuously and repeatedly measured a plurality of times. To this end, Japanese patent laid-open No. 10-169493 discloses technology for repeatedly measuring with a time from when a target air-fuel ratio has inverted from lean mixture to rich mixture, until a detected value for the air-fuel ratio sensor crosses a target air-fuel ratio (rich side target value) as a diagnostic value, and when the diagnostic value exceeds a specified value (abnormality determination value) twice in succession, or when a diagnostic value for a specified proportion exceeds the abnormality determination value, an air-fuel ratio sensor abnormality is determined.
On the other hand, with this type of abnormality diagnosis, a prerequisite is that a steady state of the engine be maintained continuously, and if a transient state such as acceleration or deceleration or the like intervenes at the time of measuring the diagnostic value it is difficult to acquire a correct diagnostic result due to this disturbance.
Therefore, with technology for measuring a diagnostic value a plurality of time at the time of abnormality diagnosis, as with the technology disclosed in the patent publication described above, there are limited opportunities where abnormality diagnosis can be executed, and there is concern that it is difficult to ensure sufficient frequency of diagnosis.
The present invention has been conceived in view of the above described situation, and an object of the invention is to provide an abnormality diagnosis device for an air-fuel ratio sensor that can acquire highly precise diagnosis results without lowering frequency of diagnosis.